In a diesel fuel internal combustion engine, air is introduced into each combustion chamber (cylinder) during an air intake stoke, then compressed during a compression stroke. The compression increases the air temperature so that when diesel fuel is introduced into the cylinder at the top of the compression stroke the diesel fuel vaporizes and ignites in a process called compression ignition. The ignited fuel undergoes rapid expansion, driving the piston downward during the power stroke. Exhaust gases are expelled during an exhaust stroke and the four-stroke diesel cycle begins again.
Diesel fuel engines enjoy high reliability, primarily due to the absence of the electrical ignition system required by their gasoline powered counterparts, and generally better fuel economy than gasoline powered engines. However, in recent years natural gas has become a desirable fuel source for internal combustion engines due to its lower cost and significantly lower emissions. Large diesel engines can be converted to run on natural gas with diesel fuel as a pilot (ignition) fuel. These “dual fuel” engines require onboard natural gas and diesel fuel (dual fuel) delivery systems.
The natural gas may be stored onboard in a pressurized temperature controlled tank as liquefied natural gas, or LNG. The natural gas is introduced by a dual fuel injector into the combustion chamber, where the natural gas is ignited by the separate injection of the diesel fuel by the same dual fuel injector.
In addition to serving as a pilot fuel, the diesel fuel may be used to lubricate the natural gas side of the dual fuel injector, particularly the natural gas upper guide and needle valve seat. This requires fluid communication between the diesel fuel line and the natural gas side of the injector. To minimize the migration of natural gas from the injector back into the diesel fuel line, a pressure differential is maintained between the natural gas side of the dual fuel injector and the diesel fuel line. A hydraulic seal serves to maintain this pressure differential during engine operation, with the pressure on the diesel fuel side exceeding the pressure on the natural gas side by about 5 megaPascals (MPa).
At engine start-up and in limp-home mode conditions, the natural gas line pressure can drop well below its normal operating pressure, even to zero. A low natural gas line pressure can result in an unacceptably high flow of diesel fuel into the natural gas side of the injector and back into the natural gas line.
The present disclosure is directed toward one or more of the problems set forth above.